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The first documented use of radiation as a therapeutic treatment was for a cutaneous malignancy, in a patient with squamous cell cancer of the nose, in 1900. Over the next century, radiotherapy was widely used in the treatment of both malignant and benign disorders of the skin, in both adults and children. As the long-term consequences of radiotherapy became evident, particularly the risk of radiation-induced malignancy, its use in the treatment of children and benign diseases declined. Radiotherapy continues to have a small but important role in the management of benign proliferative diseases of the skin, but is more commonly used as a valuable adjunct or alternative to surgery for both premalignant and malignant lesions.

There are several choices for radiation modalities, some commonly available and others only in specialized centers. The selection is made on the basis of the anatomic location and size of the target, tumor biology, the nature of critical surrounding structures, and availability. In particular, with respect to the cutaneous targets, the depth of the lesion plays a large role in determining the optimal therapy.

High-energy photons, in the form of γ- or X-rays, are most commonly produced by a linear accelerator (linac) and are available in a spectrum of energies. Incident radiation deposits its energy as it passes through matter, becoming attenuated as a function of distance and the density of the tissue. Higher energy beams deliver increased dose at depth in tissue, and proportionally less at the surface. Lower energy radiation deposits dose primarily at the target surface, sparing deeper matter. The most commonly available treatment energies are in the megavoltage range, which deposit their dose at a range practical for the treatment of targets in human tissue. Such beams were designed to relatively spare surface structures such as skin, which would otherwise be dose limiting, in the interest of delivering higher dose to deeper target structures. This is in contrast to more superficial radiation energies, which are often more appropriate for cutaneous targets. Depth dose curves demonstrating the absorption of X-rays as a function of their energy are demonstrated in Fig. 240-1A. Most linear accelerators in clinical use provide radiation in the range of 6–18 megavolts (MV).1